Members of the ErbB receptor Tyr kinase (RTK) family have been identified as targets of ionizing radiation (IR) in autocrine growth regulated human mammary and squamous cell lines. The IR- induced activation of ErbB1, reflected in increased Tyr phosphorylation, is indistinguishable from that of EGF, a physiological ligand of ErbB1. IR-induced activation of ErbB1 results in secondary activation of existing signaling cascades including the MAPK and JNK pathways. MAPK activation can be linked to a cytoprotective proliferation response whereas stimulation of JNK exerts a cytotoxic effect. The proposed mechanistic studies focus on the principal differences of ErbB activation by growth factors (GFs) and IR. GFs induce a hierarchic activation profile depending on their specificities for different ErbB RTKs; in contrast, IR activates all ErbB species in a non-discriminatory way. Since the dimerization patterns of ErbB1-4 can mediate different signals, the hypothesis that varied stimulation by GFs and IR results in different cellular responses will be tested. Responses to IR are predicted to be much more dependent on relative ErbB expression levels known to vary substantially in different human carcinoma cells, the ultimate therapeutic target of IR. The overall research objective is addressed in three inter-related aims of independent validity. Specific Aim 1: The interactions, i.e. dimerization profiles critical for ErbB RTK activation by GFs and IR, will be characterized in detail. Immunochemical studies will be conducted with cell constructs in which the expression levels or functions of ErbB proteins can be selectively modulated through induction of ErbB-antisense sequences or tyrphostin inhibitors. Specific Aim 2: Studies will focus on protein tyr phosphatases (PTPs) in regulation of basal ErbB1 Tyr phosphoryation. PTPs are inhibited by reactive oxygen-mediated oxidation of a catalytic domain Cys resulting in increased ErbB Tyr phosphorylation. Experimental approaches include cellular redox modulation and expression manipulation of cellular PTPs. Specific Aim 3: The biological consequences of different dimerization patterns after IR and GF exposures of cells will be quantified. Following our hypothesis that defined dimers can affect intracellular signalling cellular responses will be characterized at two levels. As primary effectors, PLCgamma and PI3K both interacting with ErbB proteins will studied. This will by followed by establishing activation profiles of secondary effectors MAPK, JNK, and Akt which control cytoprotective and cytotoxic responses.